The enzyme .beta.-galactosidase, and its derivatives, are used in innumerable assays. .beta.-galactosidase is able to hydrolyze multiple substrates. The choice of substrate for an assay involving an enzyme with .beta.-galactosidase activity (EC 3.2.1.23) is contingent upon the conditions under which the enzymatic reaction takes place and the conditions under which the concentration of reaction products must be determined. Some of the more important factors to consider when selecting a substrate for a given assay include solubility, stability, affinity for the enzyme, speed of the reaction, strength of the signal generated, and the absorbance wavelength at which the assay results are determined. As the variety of assays that involve measuring .beta.-galactosidase activity increases, the need for more substrates with different properties will also increase because the constraints placed upon the substrates will also increase.